This application relates generally to the field of information storage and more particularly to a method and apparatus for verifying that data written on a storage disc can be reliably recovered during subsequent read operations.
The need for larger capacity data storage devices has become critical with the staggering pace of advances in computer technology. The most common data storage device used within computers today is the disc drive. The amount of data that can be stored on a disc drive has increased dramatically in recent years. Coupled with the need for larger storage capacity is a desire to increase the information throughput of the drive (i.e., increase the rate at which information is stored to and retrieved from the disc drive).
Most disc drives are composed of one or more magnetic media discs attached to a spindle. A spindle motor rotates the spindle and discs at a constant high speed. An actuator assembly adjacent to the disc(s) has actuator arms extending over the discs, each with one or more flexures extending from each actuator arm. A read/write head is mounted at the distal end of each of the flexures. The read/write head includes an air bearing slider enabling the head to xe2x80x9cflyxe2x80x9d in close proximity above the corresponding surface of the associated disc. The distance between the read/write head and the surface of the associated disc during disc drive operation is called the xe2x80x9cfly height.xe2x80x9d Information is stored on and retrieved from a disc via the read/write head.
Information is stored on the disc surface as a bit. A bit is represented by a xe2x80x9c1xe2x80x9d or xe2x80x9c0,xe2x80x9d which corresponds to a change or lack of change, respectively, in the orientation of adjacent magnetic domains on the disc surface. A domain""s magnetic orientation is changed using the disc drive""s write element. A write element is essentially an inductive coil. A magnetic field is generated around the write element by passing a current through the coil. The magnetic flux of the generated field, if strong enough, orients the magnetization direction of a magnetic domain located on the disc surface. The direction of the current in the write element dictates the direction of the magnetic flux of the generated field, and subsequently, the orientation direction of the magnetic domain.
As mentioned above, the strength of the magnetic field present at the disc surface must be strong enough to orient the magnetic domain. The strength of the magnetic field relative to the disc surface decreases as fly height increases. The magnetic field relative to the disc surface may not be strong enough to change the magnetic domain""s orientation if the fly height becomes too great. One solution is to increase the strength of the magnetic field. The strength of the magnetic field, however, must be limited to prevent changing the orientation of adjacent domains located on the disc surface. The fly height of the read/write head, therefore, is critical to insure that the generated magnetic field is sufficient to change the orientation of the desired magnetic domain without changing the orientation of adjacent magnetic domains.
Information is retrieved from the disc surface using the read element. The read element senses orientation changes of the magnetic domains on the disc surface. The changes in the magnetic domain orientations create an electrical signal in the read element. The read element must be very sensitive to detect the orientation changes of the small magnetic domains. The disc drive""s preamplifier is used to amplify the resulting signal before the signal is sent to the disc drive controller. Again, the fly height of the read/write head is critical to insure that the read element is close enough to the disc surface to detect the orientation changes in the magnetic domains such that an electrical signal is produced within the read element.
Each disc is radially divided into a finite number of concentric tracks to facilitate organization of the stored bits. Each track is a certain width and is separated from the adjacent tracks by a xe2x80x9cblank space.xe2x80x9d This blank space prevents information stored in one track from overlapping the information stored in an adjacent track. The number of tracks located on each disc surface is known as the xe2x80x9ctrack density.xe2x80x9d Each track is linearly subdivided into sections, called segments. Bits are written to and read from these segments by the read/write head. The linear density of bits stored within each segment is called the xe2x80x9cbit density.xe2x80x9d
The product of track density and bit density is known as xe2x80x9careal density.xe2x80x9d The recent trend being followed by disc drive manufacturers is to increase the recording media""s areal density so that the amount of data stored can be increased without increasing the physical size or the number of discs used in a drive. For example, the areal density of early disc drives was less than 1 gigabits per square inch (Gbits/sq. inch), whereas today, disc drives with areal densities greater than 40 Gbits/sq. inch are being tested. Manufacturers increase areal density by increasing both track density and bit density. Track density is increased by narrowing the track width and/or narrowing the width of the blank spaces between tracks. Bit density is usually increased by increasing the recording speed in order to record higher frequency bits. A higher frequency bit is smaller, and therefore, takes up less space on the disc surface.
An increase in areal density has a direct effect on the fly height of the read/write head. The write element must fly closer to the disc surface when writing information at higher areal density because the xe2x80x9cblank spacexe2x80x9d and track width become smaller. A decrease in fly height is necessary to insure that the magnetic field present at the disc surface is strong enough to change the desired domain""s orientation without overwriting information stored in an adjacent track. Likewise, the read element must fly closer to the disc surface when retrieving information from a disc with higher areal density because the smaller bits generate a smaller magnitude signal within the read element. The fly height, in summary, must become smaller in order for the read and write operations to be completed effectively as areal density increases.
The fly height in current disc drives has decreased to less than 1 microinch (xcexc-in). A small contaminate particle, vibration, external shock, or a disc surface defect, among others, can affect disc drive performance at such low flying heights. For example, a dust particle that hits the read/write head can cause the read/write head to xe2x80x9cbouncexe2x80x9d away from the surface of the disc. If this bounce occurs while information is being written to the disc, the magnetic field generated by the write element may not be strong enough, relative to the disc surface, to change the desired domain""s orientation and accurately record the information on the disc. This problem is known in the art as a xe2x80x9cskip writexe2x80x9d or xe2x80x9cskip write error.xe2x80x9d
Most disc drives are manufactured in a clean room environment in order to prevent the presence of contaminate particles in an assembled disc drive. Most clean rooms are Class 100 clean rooms. Class 100 means that 100 contaminate particles per-liter-of-air are present in the room. Class 100 clean rooms were adequate for older disc drives with higher fly heights, but current disc drives require Class 10 clean rooms. Class 10 means that only 10 contaminate particles per-liter-of-air are present in the room. The amount of filtering needed to reach and maintain Class 10 status dramatically increases the cost of the disc drive manufacturing process.
Disc drive manufacturers place filters within the disc drive to trap the contaminate particles introduced during the manufacturing process. The filters also trap contaminate particles emitted from the drive""s components during normal operation. The filters require between 100 and 200 hours of normal drive operation to effectively capture the contaminate particles. A brand new drive, however, is usually used by industry testing facilities during benchmark testing (i.e., performance testing); before the internal filters have had an opportunity to trap the contaminate particles. The likelihood that a contaminate particle will cause a skip write during benchmark testing is high. The detection and correction of a skip write error during benchmark testing adversely affects the disc drive""s performance rating. Any degradation of performance during benchmark testing can lead to a decrease in consumer demand for the particular disc drive being tested. Additionally, detection and correction of a skip write error during normal disc drive operation adversely affects the user""s computing efficiency.
Disc drive manufacturers address skip write problems by using read verification and/or fly height monitoring. Read verification consists of reading the information that was stored during the write operation and comparing the information actually stored to the information intended to be stored. If the information actually stored differs from the information intended to be stored beyond an acceptable level, a likely skip write error is detected. A subsequent write operation is completed when a skip write error is detected and another read verify operation is completed to insure that the subsequent write operation was successful. This process continues until a successful write operation is completed at that disc location, or the information may be stored at a different disc location after a predetermined number of write attempts have failed. The additional read, compare, and write steps of the read verify operation, however, take a large amount of time and adversely affect the performance rating of the disc drive.
A second method of determining whether a skip write error has occurred is fly height monitoring. For example, the fly height of the write element can be monitored during normal write operations by determining the ratio of the magnetic pulse area to the magnetic pulse peak. An increase in the flying height of the read/write head corresponds to an increase in the magnetic pulse area, and therefore, an increase in the measured ratio. The writing process can be suspended and recovery procedures can be instituted if the ratio between the magnetic pulse area and magnetic pulse peak indicates that an acceptable fly height has been exceeded. Such a method requires complicated computations to determine the magnetic pulse area, the magnetic pulse peak, and the ratio between the area and peak.
Accordingly, there is a need for an apparatus and associated method of detecting whether a successful write operation has been completed and of correcting for an unsuccessful write operation that does not require a read verification procedure or complicated fly height computations.
Against this backdrop, embodiments of the present invention have been developed to determine whether a successful write operation has been completed. Embodiments of the present invention offer an apparatus and associated method to monitor the fly height of the read/write head during a write operation. The fly height can be used as an indication of whether the write operation was completed successfully. Embodiments of the present invention can be used for various types of storage systems such as magnetic and optical disc drives among others, however, a magnetic disc drive has been used to illustrate an embodiment of the present invention.
Accordingly, a preferred embodiment of the present invention relates to an apparatus and associated method monitoring the fly height of a read/write head to determine whether a skip write error is occurring. The fly height is determined by monitoring the parasitic capacitance between the write element and the disc surface. The amount of parasitic capacitance corresponds to the distance between the write element and the disc (i.e., the fly height). A change in parasitic capacitance causes a frequency change in an oscillator of a test circuit. The amount of parasitic capacitance is determined and compared to a predetermined threshold level to ascertain whether the fly height remains within an acceptable range, or a skip write error is occurring. The write operation is suspended when a skip write error is detected and a rewrite procedure is instituted.
Thus, a preferred embodiment of the present invention relates to an apparatus and associated method for determining the fly height of a read/write head by monitoring the parasitic capacitance present between the read/write head and a disc surface. The apparatus includes a test circuit, connected to the closed-loop write circuit created by the write element and the write element electrical leads, to determine the parasitic capacitance and to compare the amount of parasitic capacitance to a predetermined threshold value. The test circuit includes an oscillator, a frequency modulation detector, a filter circuit, a sample-and-hold circuit having a time constant, a comparator, a threshold trigger level circuit, and various capacitive and inductive elements among others. The test circuit is sensitive to frequency changes caused by a change in the amount of parasitic capacitance between the write element and the disc surface. The test circuit uses the change in frequency to determine the amount of parasitic capacitance, and ultimately, the fly height of the write element. The test circuit compares the fly height of the write element to a predetermined threshold to determine whether a skip write error is occurring.
Embodiments of the present invention do not require a read verification step to be completed, nor do embodiments of the present invention require complicated computations to determine whether a skip write error has occurred. Therefore, embodiments of the present invention increase the disc throughput; in other words, the speed that data can be stored and retrieved from the disc. Furthermore, embodiments of the present invention immediately achieve the increase in throughput; eliminating the 100 to 200 hours of operation usually needed by the disc""s internal filters to trap contaminate particles. Therefore, the need for more stringent clean room filtering can be reduced because acceptable performance can be achieved even though higher contaminate levels might be present in the disc drive.
These and various other features as well as additional advantages which characterize embodiments of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.